Sorting an ArrayList of Person with java collections

Question

Below the code I am using works just fine and outputs the names except that the sort method is not working. I expected "Collections.sort(nameFromText);" to sort the ArrayList in alphabetical order by first name.

What am I doing wrong?

public static void main(String[] args) throws IOException {
    // Create and populate text file
    Writer textFile = new FileWriter("names.txt");
    String[] nameArray = new String[] { "Tina Tully\n", "Bill Simpson\n",
            "Dana Smith\n", "Ralph Andrews\n", "Greg Smithers\n",
            "Lisa Krump\n", "Gill Bitters\n", "Barbara West\n",
            "Sandra McDonald\n", "Bart Willis\n", "Bucky Zimmerman\n",
            "Richard Vicks\n", "Velma Tarp\n", "Winslow Tunnell\n",
            "Andrew Letterman\n", "Betty Trump\n", "Waldo Smith\n",
            "Kyle Ronno\n", "Vivian West\n", "Wendy Tunnell\n" };
    generateText(textFile, nameArray);

    // Create object of previously created text file
    Scanner pullFile = new Scanner(new File("names.txt"));

    // Create 20 Person Objects and add to ArrayList data structure with
    // name variables assigned to values from text file
    ArrayList<Person> nameFromText = new ArrayList<Person>();
    fillArrayList(nameFromText, pullFile);

    // Sort ArrayList
    Collections.sort(nameFromText);

    // Print ArrayList
    printNamesFromObjects(nameFromText);
}

private static void printNamesFromObjects(ArrayList<Person> namesFromText) {
    for (int i = 0; i < 20; i++) {
        System.out.println(namesFromText.get(i).name);
    }
}

private static void fillArrayList(ArrayList<Person> nameFromText,
        Scanner pullFile) {
    while (pullFile.hasNext()) {
        Person obj = new Person(pullFile.nextLine());
        nameFromText.add(obj);
    }
}

private static void generateText(Writer textFile, String[] nameArray)
        throws IOException {
    for (int i = 0; i < 20; i++) {
        textFile.write(new String(nameArray[i]));
    }
    textFile.close();
}

You'll find this useeful. thejavageek.com/2013/06/17/sorting-user-defined-objects-part-1 and thejavageek.com/2013/06/17/sorting-user-defined-objects-part-2 — Prasad Kharkar
– Prasad Kharkar, Commented Sep 28, 2013 at 6:53

Community · Accepted Answer · 2016-06-25 11:51:42Z

Collections.sort(List<T>) method expects the element of the list it is sorting to be comparable. Either the element type T should implement the Comparable interface, or you should use the overloaded sort() method, that takes a generic Comparator instance.

In the code below, you are satisfying neither of the above conditions. Neither your Person class implements Comparable, nor you are passing any Comparator instance.

ArrayList<Person> nameFromText = new ArrayList<Person>();
fillArrayList(nameFromText, pullFile);
// Sort ArrayList
Collections.sort(nameFromText);  // How to sort?

You should create a Comparator for your Person class to tell the sort() method how to sort it (may be on String stored in Person class)

Here's how you implement a generic comparator:

public class PersonNameComparator implements Comparator<Person> {
    public int compare(Person p1, Person p2) {
        return p1.getName().compareTo(p2.getName());
    }
}

And then your Collections.sort() method invocation should be like: -

Collections.sort(nameFromText, new PersonNameComparator());

@DavidTunnell Your're welcome.. You can also google - "Difference between Comparator and Comparable` to get more information.. Comparable is also used for this purpose.. But Comparator is wider, as you can create multiple sorting logics..
I think you have a typo on your code. Shouldn't it be: ... (Person) o 1 ; ... Person p2 = (Person) o 2 ; ?

Satheesh Cheveri · Accepted Answer · 2012-10-07 18:45:45Z

4

Alternatively you can implement Comparable interface directly in Person class and override 'compareTo(Object obj) ' method. In this case you would not need to create new class for comparator. And this behaves like inbuilt sorting.

answered Oct 7, 2012 at 18:45

Satheesh Cheveri

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2 Comments

Rohit Jain Over a year ago

+1 Thats a valid point.. But you should prefer Comparator over Comparable. That way you can have multiple sorting logics for your class..

Satheesh Cheveri Over a year ago

Yes, as comparator give you the flexibility when there are more than one sorting candidates.

Patrick Kostjens · Accepted Answer · 2013-08-26 19:25:26Z

2

Try this:

List<String> inputString =  Arrays.asList("Sijan", "Sudeep", "Parasar", "Raj Kumar");
Collections.sort(inputString);
System.out.println(inputString);

edited Aug 26, 2013 at 19:25

Patrick Kostjens

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answered Aug 26, 2013 at 18:57

MudvariSijan

311 bronze badge

Comments

Divyesh Kanzariya · Accepted Answer · 2019-01-09 12:52:44Z

1

If you using Java 8 then use one liner Lambda Expression

ArrayList<Person> nameFromText = new ArrayList<Person>();
fillArrayList(nameFromText, pullFile);
nameFromText.sort((p1, p2) -> p1.getName().compareTo(p2.getName()));

answered Jan 9, 2019 at 12:52

Divyesh Kanzariya

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Comments

Antesh Sharma · Accepted Answer · 2013-11-21 06:22:49Z

use Collections.sort(managerNameList);

ArrayList<String> managerNameList = new ArrayList<String>();

    managerNameList.add("antesh");
    managerNameList.add("Lalit");
    managerNameList.add("Gokul");
    managerNameList.add("Ajay");

    System.out.println("Arraylist before sorting");    
    for(String name: managerNameList)
    {    
        System.out.println(name);           
    }

    Collections.sort(managerNameList);

    System.out.println("Arraylist after sorting");    
    for(String name: managerNameList)
    {    
        System.out.println(name);           
    }

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